Managing wild geldings on public lands

The vast majority of domestic male horses in the United States are “geldings,” or neutered/castrated males. Horse owners normally choose to geld their male horses for a variety of reasons, some of which are related to improving behavior in a captive setting. Likewise, the BLM also typically gelds wild horses that are removed from the range and offered for adoption, but has usually not released geldings back into the wild, except in a few herds. 

A new peer-reviewed article affirms that having some wild geldings in wild horse herds is safe and does not disrupt wild horse welfare or social systems. However, unless a high fraction of males are gelded and released, their presence does not meaningfully reduce mare fertility rates in the long term. 

In the study, Colorado State University researcher Sarah King and her coauthors from the US Geological Survey studied horses at the Conger Herd Management Area in Utah. About 40% of the male wild horses in the Conger Herd Management Area were gelded, with the other 60% remaining fertile. The main objectives of the BLM-funded study were to monitor the welfare effects of gelding, to test for any differences in behavior or habitat use that resulted from having some geldings in the herd, and to test for effects on survival and foaling rates. A previous 1992 study by Garrott and Siniff showed that approximately 80% or more of a herd’s stallions would need to be gelded to substantially reduce herd growth rates.

Pre-treatment observations began after gathers in 2016. A fraction of the stallions at Conger Herd Management Area were gelded after a second gather in 2017. Horses at the nearby Frisco Herd Management Area had no geldings, and were observed as a control group for comparison. Horses were individually marked with numbered freeze brands, and some were radio collared or carried telemetry tail tags, to help researchers locate them. 

Having some geldings in the herd did not appear to change mare behaviors. Social bands continued to consist of mares and their offspring with one or two adult males, and separate groups of bachelors, as is typical of other wild horse herds. Likewise, wild geldings’ behaviors and body condition were largely the same as those of fertile stallions, other than some decrease in reproductive and marking behaviors. Unpublished results from the study also indicate that movements by wild geldings and fertile stallions were similar. 

The observed decrease in mare fertility rates was moderate and only detectable in the first year after the gelding treatment. Though some geldings were still associated with mares even three years after treatment, geldings that started the study with harems tended to have smaller harem size over time, or lost harems entirely.

This is an example of how the BLM supports research that could improve its management and understanding of wild horses and burros on the public lands. Though the BLM does not typically geld a large percentage of a herd, another way that geldings can also reduce overall herd growth rates is if the overall fraction of males is more than females, with geldings taking the place of some mares. This study confirms that gelding male horses and releasing them back to the range is safe for these cherished, federally protected animals.